This invention relates to a quadrupole ion trap and method, and more particularly to an ion trap in which shim electrodes compensate for electric potential faults introduced by apertures drilled into the entrance and exit end caps.
An ion trap, in its most common configuration, is composed of a central ring electrode and two end cap electrodes. Other quadrupole ion trap configurations are described in U.S. Pat. No. 5,420,425. Generally, each electrode has a hyperbolic surface facing an internal volume known as the trapping volume. The trapping volume also serves as an analyzing space in which selected ions are retained and sequentially ejected, based upon their mass and charge. It also serves as a reaction volume, in which fragmentation of charged particles is caused by both collisions and interactions with specific fields. When a radio frequency (RF) voltage is applied between the ring and end cap electrodes, a quadrupolar potential is induced within the trapping volume. Generally, each of the end caps has one or more holes drilled into the center for the purpose of introducing ions or electrons into the trapping volume through the entrance end cap and for ejecting ions from the trapping volume to an external detection system through the exit end cap. Ions introduced into or formed within the trapping volume will or will not have stable trajectories, depending upon their mass, charge, the magnitude and frequency of the applied voltages, and the dimensions and geometry of the three electrodes.
Quadrupole ion trap potentials deviate from the ideal quadrupolar potential for two reasons: 1) because of holes drilled into the end caps, and 2) because the shapes of the electrodes have finite values. These effects are referred to as electric potential faults.
The electric potential deviation results in both peak broadening and, in some cases, a shift in measured ion mass from the theoretical mass values. Several schemes have been used and proposed to neutralize electric potential fault effects upon motion of the trapped ions. Franzen et al. U.S. Pat. No. 5,468,958 describes a quadrupole ion trap with switchable multipole fractions, which can be used to correct the electric potential errors due to the finite size of the electrodes.
Electric potential deviations due to the finite size of the trap electrodes are relatively insignificant compared to the deviations caused by the holes used to inject and eject ions. One method for correcting the deviations due to the holes is to stretch the spacing of the end cap electrodes from the ring electrode beyond the theoretical spacing predicted by solving the equations of motion of charged particles contained within the trapping volume
A different approach has been taken by Shimadzu Corporation in U.S. Pat. No. 6,087,658, in which they have mechanically modified the end cap electrodes with a bulge at the internal end of each hole. The stated purpose of the bulge is that it corrects the deviation in the electric potential from the pure quadrupole electric potential by controlling the deviation of the electric potential around the central end cap hole.
It is an object of the present invention to provide a quadrupole ion trap in which electric potential faults are minimized.
There is provided a quadrupole ion trap of the type including a ring electrode and first and second end cap electrodes which define a trapping volume. The end cap electrodes include central apertures for the injection of ions or electrons into the trapping volume and for the ejection of stored ions during analysis of a sample. Electric potential faults in the RF trapping potential are compensated by shim electrodes carried within the central apertures and electrically insulated from the end cap electrodes.
In another embodiment of the invention, there is provided a linear quadrupole ion trap with four electrodes, each divided into one or more sections. One or more apertures are provided for ejection of ions during sample analysis. Electric potential faults in the RF trapping potential are compensated by shim electrodes carried within the apertures and electrically insulated from the adjacent electrodes.